1. Field of Invention
This invention relates to electron gun apparatus for providing an electron beam for use in a visual display cathode ray tube, and more particularly to a modulated electron gun apparatus for providing a hollow electron beam whose average beam diameter may be modulated over a determined range to provide a plurality of different discrete average diameter values within the range.
2. Description of the Prior Art
In certain video display systems having a cathode ray tube (CRT), it is desirable to provide excitation of each of a plurality of different phosphor groupings disposed on a faceplate of the CRT in a concentric, determined geometric pattern, such as that shown and described in the hereinbefore referenced, copending application. This phosphor grouping precludes the use of a conventional solid electron beam, but requires a hollow electron beam capable of providing a plurality of suitable average diameters to excite the selected one of the concentric phosphors without impinging on adjacent phosphors. Similarly, the electron beam must be capable of providing different discrete diameter values in dependence on the diameters of the various concentric phosphor groupings.
In general, electron gun assemblies capable of providing hollow electron beams are known in the prior art, and have been used for a number of years in traveling wave tubes, where a hollow electron beam is propagated through a helix which provides an RF modulating signal to the beam to modulate the electron flow and provide a determined signal gain. In these applications, however, the use of the hollow electron beam is generally required only to limit the power dissipation within the traveling wave tube, since the applied RF signal influences the displacement of only the electrons on the outer surface of the beam and not those on the central portion, the internal electrons represent wasted power. Therefore, in a high power traveling wave tube it is preferred to use the hollow beam to limit the power dissipation and prevent burn out of the anode element. Similarly electron gun structures for providing modulated, hollow electron beams have been used in high power x-ray tubes to limit the tube power dissipation, such as that described in U.S. Pat. No. 3,892,989 to N. M. Gralenski, and entitled "Convergent Flow Hollow Beam X-Ray Gun Construction", where a hollow beam is generated and then modulated continuously by an applied AC signal to limit the high power concentration which would be provided by a solid electron beam, while still maintaining the required high x-ray emission.
At the present time, there is no known electron gun apparatus in the art for providing a hollow electron beam with a plurality of discrete diameter values provided by beam modulation over a determined modulation range and which are suitable for use in a cathode ray tube structure of the type described in the hereinbefore referenced, copending application.